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Thursday, 6 December 2012

How to Adjust Ducati Desmo Valves

This was an introductory guide I wrote a few years ago about adjusting 4V (specifically, Desmoquattro) valves on a Ducati.

Often demonized and rarely understood,
desmo valve adjustment is becoming a bit of a lost art as people are
discouraged from touching their own machines because of the supposedly
frightening complexity of desmodromic valves and the need for “special tools”
to do the job. That and the fact that current generations of Ducatis have 15 000 mile plus valve adjustment intervals (which, by the way, was not dictated by any great leap forward in technology - they simply arbitrarily raised the intervals to alleviate the stigma of frequent servicing. The system remains more or less unchanged from before.) Be afraid, the ignorant man says - Desmo valves are witchcraft that are not to be understood by mortals.

But the truth is desmo valves
are no harder to adjust than traditional valves, the only difference being that
there is twice the number of… everything. Double the shims, double the
measurements, double the rockers, etc. But with a little patience and the right
techniques it’s all very straightforward and not as mind boggling as some
people would want you to think.

Opening note

I’m doing this adjustment with the head off
and on the bench. This is the easiest way to adjust four valve heads, but has some
problems – first, you need to replace the head gaskets, and second you need a
special wrench to take the heads off. On pre-1999 bikes, head gaskets are
thick, fibre-composite items that cost about 25-30$ each (so 60$ total). This
makes taking the heads off worthwhile - if you can find those old gaskets, they are getting hard to source nowadays. On post-1999 models (starting with the
996) a thinner, multilayer metal gasket was used – these run about 125-150$
each. Obviously if you are running a tight budget and have a later bike, leave
the heads on and adjust them in place. I will not cover the procedure for
removing the heads; this tutorial assumes you have the heads off, or at least
have the timing belts removed with the heads in place.

If you take the heads off, it’s recommended
you lap the valves with grinding compound. Ideally this should be done every
6000 miles (10 000 kms) for optimum sealing.

HA. HA. Cough.

Every 12-24K miles (20 000 – 40
000 kms) is good enough for most people.

Also think about setting your piston
squish while you are in there, these motors respond well to careful squish
setup.

“Special Tools"

To remove the heads (if you want to, see
above) the special cylinder head wrench is a closed end 12-point 15mm with a socket attachment for a torque wrench / ratchet. You can buy cheap ones from DesmoTimes or
Motoreva, for around 40-50$. Compared to 300~ for the Ducati tool, that’s a
bargain and is well worth it if you plan on doing your own work.

You will also need a closing shim
adjustment tool – a top hat shaped plug that fits into the closer shims to
measure them with a micrometer. Aftermarket items sell for around 15$.

To remove the rocker pins, I use an M5 bolt
with a nut, a washer, and a 5/8” socket. This actually works better than expensive specialty slide hammers, in my opinion, because you can slowly and gently extract the pin as much or as little as you like (rather than yanking it out suddenly).

Checking Clearances

First, lets look at the desmoquattro setup. We
have a double overhead cam design driven by automotive-style timing belts on
the right hand side of the engine. There are two rocker arms per valve, one for
opening and one for closing. Thus on each cam there are four lobes – two for
opening, two for closing. The openers are the small lobes on the outside
operating the top rockers. The closers are the large lobes on the inside
operating the lower rockers. The closing rockers are aided by “helper”
hairsprings that close them at rest or at low rpm. You can feel the tension of
the helper springs when turning the cams or pushing on the closing rocker. At higher RPM they become pointless,
but when the engine is starting or at idle they are needed to maintain sealing
with loose closing clearances. No, it's not a good idea to remove them, unless you don't need your bike to idle.

There are two shims per valve as well – one
small shim sitting on the top of the valve stem to set the opening rocker
clearance and a large shim wrapped around the valve stem (securing the
retaining collets) to set the closing clearance.

The easiest way to check the clearances if
you leave the heads in place is to loosen and
remove the timing belts with the piston at top dead centre on whichever
cylinder you are working on. TDC cam position and the default position for
measuring the clearances is with the opening lobes (the small ones operating
the top rocker) pointing towards the top outside corner of the head.

Checking the opening clearance is simple –
at TDC, check the gap between the top shim and the rocker arm.

There are two ways to check closing
clearance, and the best method is to use both of them. First, there is
“loaded/unloaded” clearance. The “unloaded” clearance is the opening clearance
you just measured. To measure “loaded” clearance, you take a
small socket (1/4 drive) on an extension and press down firmly on the lower rocker arm (the
forked part poking out below the bottom shim). Now measure the gap between the
top shim and the top rocker arm. You will get the opening clearance with the
closing clearance added to it – the loaded gap. What you are doing is pushing
the closing rocker against the tension of the helper spring to squeeze it
against the cam, thus giving you your different measurement.

If the clearance is small you might have trouble "feeling" the rocker move, just apply steady pressure and take the measurement. Don't put too much weight into it. If it is loose you will probably feel it click slightly as it moves down.

The second measurement is the clearance
between the cam lobe and the surface of the closing rocker. This is tricky to
measure because it is hidden behind the cam, and needs a set of long, angled
feeler gauges. In a pinch you can disassemble a normal set of feeler gauges and
use the individual leaves to measure the gap. I measure the closer this way
after doing the loaded gap, as a double check for my first measurement.

So in this case on the exhaust side I had:

0.127 mm opening clearance

0.178 mm loaded gap

Subtract opening clearance (unloaded gap)
from loaded gap.

=

0.051 mm closing clearance

This is too tight on the closer side.
Ducati recommends the following for most desmoquattros:

Intake

Opening - 0.05- 0.18 mm

Closing – 0.16 – 0.25 mm

Exhaust

O - 0.05 – 0.23

C – 0.11 – 0.20

So this means we have to open things up to
replace the closing shim.

NOTE: I am using the Ducati specs for most Desmoquattro models (barring SPS, SP, and R). Everybody and their dog has a different set of specs. For closing clearances I've heard anything between zero and 0.25 mm and everything in between as being "the best". When in doubt, ask an expert. I'm not one, so I go by the manufacturer's spec sheet.

- Changing Shims -

Replacing opening shims is a simple
process, but changing closing shims is a much more involved procedure and has a
number of steps. I will detail how to replace each type of shim individually.

First, you need to know that each rocker
pivots on a pin inserted into the side of the head. There is one pin for each
rocker, with small oilways through the centre of the pins. To remove the cams, and to remove either of the shims, you need to
withdraw the pins out far enough to move the opening rocker off to the side off
the top (opening) shim.

To withdraw a pin, you first need to take
the central side covers off the heads. These are the triangular covers located
on either side of the head between the cam end caps. On the left side, there is
an external oil line that needs to be disconnected before removing the covers.

You will see four pins on each side of the
head, the lower ones are for the closing rockers (and should be left alone) the
upper ones are for the openers. Place the socket over the pin and extract it
with the bolt and nut as shown; be sure to have a long bolt so you can screw it
fully into the end of the pin, otherwise you risk stripping the inner thread of
the pins.

Slowly tighten the nut to pull the pin out;
it will feel tight at first but will gradually loosen as the pin is extracted.
As the pin comes out, you will notice that you can slide the opening rocker arm
sideways. Keep pulling the pin until you can slide the rocker off the top shim
and to the side. You don’t have to remove the pin completely unless you want to
remove or replace the opening rocker.

Now you can remove the top shim. If you are
only changing the opening shims, this is all you have to do. Once you replace
the shim, reposition the rocker arm over the shim and pound the rocker pin back
into position with a hammer.

The camshaft is removed for clarity, but if you are only doing the opening shim you don't need to remove the cam.

To replace a closing shim, things are a bit
more complicated.

First, you need to remove the camshaft. To
do this, you need to withdraw the rocker pin of the opening rocker on the right
hand side so you can slide the rocker off the shim. Once this rocker is moved,
you can manoeuvre the cam out. Undo the two retaining bolts on the right hand
pulley cap and firmly pull the assembly out, being careful to turn the cam
lobes so they don’t get caught on the rocker arm.

Now you can see the rocker arm faces
clearly. Check the cam surface of the rockers for wear or signs of chrome
flaking, a common and serious problem on any post-1995 4V Ducati. Any flaking
rocker arms need to be replaced with improved aftermarket items, available from
MBP Ducati or Megacycle in exchange for old rockers. If not, you risk damaging
the cams, throwing off the valve clearances, and sending chrome flakes into the
oiling system. In the case of this head, the rockers are showing the early
signs of flaking, with dull grey patches along the cam contact surface. "Good" rockers should have smooth burnishing, and a slight high polish sheen along the contact area. Once the rockers flake it's obvious - the face will be chipped and rough. Catch them early and you won't do serious damage. But neglect them and they will eat the surface of the cam lobe and throw metal swarf into the oil passages. That's bad.

A closing rocker that has flaked. That's bad.

If money is no object, Electraoen makes solid steel rocker arms that have no chrome surface at all, just solid hardened metal. It's a wet dream for anyone who has had to replace rockers... But the cost is prohibitive. About 2650$ for a complete set of 16 rockers. That's comparable to a full set of OEM parts, but resurfaced rockers are about half that or less (and it would be extremely unlikely you'd need to replace all 16 at once).

To remove the lower shim, use a flat head
screwdriver to lever the closing rocker down. Gently push on the edge of the
closing shim to move it down and expose the retaining collet. Use a magnet to
extract the two halves of the collet and then slide the closing shim off the
valve stem.

Please note if you are removing the closing shims with the heads in place on
the bike, you need to secure the valve stem so the valve doesn’t drop into the
cylinder. The easiest way is to wrap a zip tie tightly around the stem above
the oil seal, and be careful not to push down on the top of the stem. Even better is a pair of locking forceps with rubberized tips. And always, always make sure the piston is at top dead centre for whichever cylinder you are working on! If the valve drops onto the piston at TDC, it won't fall into the barrel completely. But if the piston is lower than TDC, it will - and that means taking the head off to extract the bugger.

The half-ring collets are pretty
unimpressive, and they are the reason for the frequent adjustment intervals of
Ducati engines. The stock half rings deform and crush easily, and will often
shatter at sustained high rpms, so don’t be too shocked if your collets come out in more
than two pieces. A solution is to get larger, hardened collets from MBP or EMS,
but on the 4V this requires using specially machined opening shims. It’s an
expensive process to install the upgraded collets and shims, but it will pay
for itself by allowing you to go from adjustments ever 6 000 miles (10 000 kms)
to every 12-18 000 miles (20 000 – 30 000 kms). You should still inspect every 6k though, especially if rocker arm flaking is a possibility.

Ducati recommends replacing the half rings
whenever you remove them. Most people agree that re-using the already deformed
half rings will make it less likely for the clearances to shift. If your half
rings aren’t shattered or obviously damaged, consider re-using them. Supposedly
the best way to re-install the rings in the same valve they came out of, so note which set goes with which valve as you disassemble.

Reinstallation is the reverse of
disassembly, as they say. One key thing is to make sure the half-rings are
properly seated – there is a “special tool” for this, but as with most special
tools you can do without it. Reinstall the half rings and closing shim the same
way you removed them, then snap the rocker arm against the shim by levering it
down and yanking out the screwdriver with the arm under moderate tension (about
½ inch of travel). This will snap the shim into place and seat the collets
snugly. To double check, make sure the top of the valve stem is flush with the
top of the closing shim, and gently lever down the rocker arm again to make sure
the shim doesn’t slide down too easily.

For MBP collets, special
opening shims are needed that are machined to accommodate the extra
height of the collets. You can buy these from EMS Ducati (see below).
They also require a more careful installation. I installed a set in my
916 at my 30 000 mile service. The fit is extremely tight compared to
the stock half rings (which immediately shows the advantage, once they
are locked in they do not move at all) so you need to take care to seat
them fully otherwise you won't get proper clearances. I recommend using a
wooden dowel and hammering the face of the closing rocker to snap the
shim over the collets. A few solid whacks are enough to seat them. Once
they are on, they are a bitch to remove - it's way easier to install
them with the heads off, as you WILL have to install and remove them a
few times while you dial in the clearances.

If you do use new half-rings, you may have
to redo the clearances from scratch. Hence why most people are happy to reuse
the half-rings.

Always double-and-triple-check the clearances on
reassembly. If anything is off, fix it. The cost of being lazy and ignoring
improper clearances is very high. When run with improper clearances, Ducatis
will shatter collets, flake rocker arms more rapidly, damage the valve seats,
and mushroom the valve stems. They can also break and drop valves, as the valve
is designed with a welded carbon-steel length on the top of the stem where the
shims and collets sit – guess where they are most likely to break under stress.

Hope this helps demystify the process a little. Try not to break anything while you are futzing around in there. I accept no responsibility for you breaking, grenading, or otherwise dun-blowing-up your ride. Please feel free to treat this as a light introductory guide - there are more detailed FAQs and manuals out there that I highly recommend you read before diving in (see below in the link section for some of my recommendations, as well as some sources for DIY parts and tools).